Boron-treated molybdenum steel



July 4, 1950 YIELD STRESS TONS PER SQ. IN.

w. E. BARDGETT 2,513,395

BORON-TREATED MOLYBDENUM STEEL Filed Aug. 5, 1947 mmurxmk MOLYBDENUM INVENTOR WILLIAM EDWARD BARDGETT ATTORNEY Patented July 4, 1950 BORON-TREATED MOLYBDEN UM STEE William Edward Bardgett, Sheffield, England, as-

signor to The United Steel Companies Limited, Sheflield, England, a British company Application August 5, 1947, Serial No. 766,398

In Great Britain July 18, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires July 18, 1966 8 Claims.

This invention relates to alloy steels and has reference to alloy steels containing boron.

It has been recognised that the addition to steels of one of the elements which can be taken into solid solution such as boron, silicon, molybdenum, chromium and nickel increases their depth hardenability and it has been suggested that the characteristic of depth hardenability can be improved by some combinations of these elements, for example a combination of boron and silicon.

The present invention, although involving alloy steels containing boron, isnot concerned with hardenability as normally understood and in fact involves considerations in the reverse direction in that its object is to provide a steel which possesses high maximum and yield stress characteristics'without introducing appreciable hardening characteristics.

Myinvention is based upon the discovery that although the addition of boron to mild steel has no appreciable effect on its mechanical properties, the addition of boron in the presence of molybdenum has a pronounced effect on the maximum stress and the yield stress without introducing marked hardening characteristics in the steel.. I

According to the present invention, therefore, I provide an alloy steel havin these desirable characteristics of high maximum stress and yield stress by the addition to mild steel of boron and molybdenum, or in other words the addition of boron to 10W carbon molybdenum steels.

A steel according to the invention possesses a high yield point in the as-rolled or normalised condition and freedom from high hardness on rapid cooling such as normally occurs in weld- Hitherto there has been difiiculty in providing a high yield point weldable steel owing to the fact that in order to obtain high yield strength, the carbon content requires to be above that which is normally safe for easy weldability. The limitation placed on the carbon content on this account does not normally permit of a yield strength greater than about 23 tons per sq. in. During welding the parent material adjacent to the weld material undergoes a cycle of heating involving rapid cooling from a temperature above the critical temperature. The properties of this Zone are very largely dependent on the hardening characteristics of the material, high hardness resulting in material which is relatively brittle and liable to crack either during the cooling operation or subsequently in service.

The improvement obtainable by the use of the invention is illustrated by the results of a series of tests on inch diameter bars which had been normalised at 930 C. of steels containing approximately 0.08 carbon, 0.42% manganese, and 0.25% silicon. The addition of 0.003% boron to a plain carbon mild steel of this composition had no substantial effect on the mechanical properties. However, when 0.003% boron was added to a steel of similar composition but containing also 0.60% molybdenum, the effect was'striking, the maximum stress being raised from 28 tons/sq. in. to 41 tons/sq.in. and the yield stress from 16 tons/sq.in. to 34 tons/sq.in. The Izod impact value of all these steels was unchanged at about 100 ft. lbs. These results are directly opposed to the general conception that boron is only of appreciable benefit in steels of medium carbon con- Zent in the hardened and lightly tempered condi- Of the low carbon, low alloy steels tested, only those containing molybdenum have been found to respond appreciably to the influence of boron in the normalised condition or normalised and tempered condition, the most beneficial results being obtained in plain molybdenum steel.

. The effect of molybdenum in low carbon, boron treated steel in the normalised condition is not linearly related to the mechanical properties. Molybdenum contents of up to about 0.20% have only a small effect on the properties, but between about 0.20 and 0.40% there is a marked increase in the yield stress and maximum stress. Beyond about 0.40% molybdenum any further effect is relatively smaller. This and other characteristics of the steels of the present invention are shown graphically in the accompanying drawing, in which the yield stress in tons per square inch of various steels is plotted against the percentage molybdenum content of the steel. The carbon and boron content of "these steels and the heat treatment they had received were as follows:

A minimum yield strength of 30 tons per sq. in. can be obtained in a boron-treated steel containingnot less than about 0.40% of molybdenum stress, but further increase in the boron content to 0.010 per cent has a markedly deleterious effect on the yield stress and also on the notch impact Value.

The major portion of Our experiments has been carried out on steels in which the boron has been introduced by the addition of a siliconcalcium-titanium-aluminium-zirconium ferroalloy containing boron, but similar results may be obtained by the use of aluminium and ferroboron with silicon contents of less than 0.1%.

The range of compositionwhich I have found to be effective for the-purpose of our invention is as follows:

Full range Preferred range Percent Percent Carbon up to .20.... 011110-015. 1 up to l 0.40 to 0.60. up to 1.0 0.10 to 0.55. 1 .20 170-. OAOtO 0 60. up to .O 0:003 1300.004

A-steel of the preferred composition in diameter bar in the normalised condition gives a minimum yield stress of .30 tons per square inch,

good ductility and toughness and freedom from excessive hardening on rapid cooling.

The freedom from excessivehardening of steel according to the invention is demonstrated by thefol-lowing figures showing the hardness acros I welds made by single runs of .mild steel electrode on normalised one inch diameter bars.

, Analyses Hardness H /lO Heat affected 0 Mn Mo B Parent Metal zone (Maximum) 0. 07 0. 37 0.61 I 0. 003 I80 219 0. l1 0. 44 0. 62 0. 003 208 252 0. 18 0. 53 0. 62 0. 0025 220 265 Another advantage possessed by the boron treated steel of the present invention is that in the normalised conditionit is generally equal to normalised and tempered material and on this account special heat treatment-following rolling is, in many cases at least, unnecessary.

Yet another advantage of the addition, in accordance with the invention, of boron to low carbon molybdenum steelis-the marked increase in ductility at elevated temperatures revealed by stress to rupture tests. As an example, the efiect of adding boron to a /2% molybdenum steel, as shown by a 24 hour test at a temperature of 1000" j R, was to increase thestress tofracture from 18 molybdenum steel having a carbon content not 4 exceeding 0.20 per cent and containing manga nese up to 1.00 per cent, silicon up to 1.00 per cent, molybdenum from 0.20. per cent to 0.70 per cent, boron'present in effective amounts but not exceeding 0.010 per cent and the remainder iron.

Y 2. A weldable high yield point boron treated molybdenum steel having a carbon content not exceeding 0.20 per cent and containing manganese up to 1.00 percent, silicon up to 1.00 per cent,

molybdenum from 0.20 per cent to 0.70 per cent, boron present in efiective amounts but not exceeding 0.010 per cent, and the remainder iron "normalised by heat" treatment.

3. A weldable high yield point boron treated molybdenum steel having a carbon content not exceeding 0.20 per cent and containing manganese up to 1.00 per cent, silicon up to 1.00 per cent, molybdenum from 0.20 per cent to 0.70 per cent, boron present in effective amounts but not exceeding .0-10 per cent, usual corrosion resisting constituents in conventional percentages, and the remainder iron. i I s 4. Aweldable high-'yiel'dpoint alloy steel having a carbon content from 0.11 to 0.15%per cent, manganese from 0: 20 to 0.6-0 per cent, silicon from 0.10 to 0.55 per cent, molybdenum from 0.40 to 0.60 per cent, boronfro'm 0.003 to 0.004 per cent, and the remainder iron.

5. Aweldable high yield point alloy'steel having a carbon content from 0.11 to 0.15 per cent, manganese from 0.40 toofiopercent, silicon from 0.10 to 0.55 per cent, molybdenum from .040 to 0.60 per cent, boron from 0.003 to 0.004 per cent, and theremainder iron and usual corrosion resisting constituents in conventional percentages.

6. A weldable high yield point boron-treated molybdenum steel having a 'oarbon content not exceeding 020 per cent andcontaining mangernese up to 1.00 per centnsilicon up to 1.00 percent, molybdenum from-0.20 per zcentr-to 0.70 per cent, boron from 0.0025 to 0.010 per cent, and the re mainder iron.

7. A weldablehigh :yield point boron-treated molybdenum steel having a carbon content not exceeding 0.20 percent and containing manganese up to 1.00 per cent, silicon up to 1.00 per cent, molybdenum from 0.20percent to 0.70 per cent, boron from 0.0025 to 0.0l0 per cent, and the remainder iron normalized by heat treatment.

8. A weldable highyield point iboron treated molybdenum steel having a carbon content not exceeding 0 .20 per centand containing imanganese up'to 1.00 per cent, silicon 11p to 1.00 per cent, molybdenum. from 0.20 percent to 0.70 per cent, boron from 0.0025 to 0.010 per cent, usual corrosion resisting constituents in conventional percentages, and the remainder iron.

WILLIAM EDWARD BARDGETT.

nnrnniinons err-ran LIN-FEED STAIES PATENTS 3 Number Name Date 2,283,299" Tis'dal'e May 19, 1942 roamongrarrnnrs Number Country Date 160.792 'G'reat 'Britain Aug. 25, 1921 506,229 Gneat Britain May'24, 1939- 50:7,545 Great Britain Feb. 1, 1940 OTHERS REFERENCES Metals and Alloys, :Qctober 19:35, 11 a gez'283. 

3. A WELDABLE HIGH YIELD POINT BORON TREATED MOLYBDENUM STEEL HAVING A CARBON CONTENT NOT EXCEEDING 0.20 PER CENT AND CONTAINING MANGANESE UP TO 1.00 PER CENT, SILICON UP TO 1.00 PER CENT, MOLYBDENUM FROM 0.20 PER CENT TO 0.70 PER CENT, BORON PRESENT IN EFFECTIVE AMOUNTS BUT NOT EXCEEDING .010 PER CENT, USUAL CORROSION RESISTING CONSTITUENTS IN CONVENTIONAL PERCENTAGES, AND THE REMAINDER IRON. 